1. Technical Field
This invention relates to methods for non-invasively determining biological tissue oxygenation in general, and to non-invasive methods utilizing near-infrared spectroscopy (NIRS) techniques for determining the same in particular.
2. Background Information
U.S. Pat. Nos. 6,456,862; 7,072,701; and 8,396,526, all assigned to the assignee of the present application and all hereby incorporated by reference, disclose methods for spectrophotometric blood oxygenation monitoring. Oxygen saturation within blood is defined as:
                                          O            2                    ⁢          saturation          ⁢                                          ⁢          %                =                                            HbO              2                                      (                                                HbO                  2                                +                Hb                            )                                *          100          ⁢          %                                    (                  Eqn          .                                          ⁢          1                )            These methods, and others known within the prior art, utilize variants of the Beer-Lambert law to account for optical attenuation in tissue at a particular wavelength. The term “absorb” in various different forms is often used to refer to light attenuation. Numerous physical phenomena may collectively cause light attenuation, including light scattering and absorptive phenomena. Relative concentrations of oxyhemoglobin (HbO2) and deoxyhemoglobin (Hb), and therefore oxygenation levels, within a tissue sample are determinable using changes in optical attenuation:
                              A          λ                =                                            -              log                        ⁢                                                  ⁢                                          (                                  I                                      I                    o                                                  )                            λ                                =                                    α              λ                        *            C            *            d                                              (                  Eqn          .                                          ⁢          2                )            wherein “Aλ” represents the optical attenuation in tissue at a particular wavelength λ (units: optical density or OD); “I0” represents the light intensity of the incident light at the wavelength (units: W/cm2); “I” represents the light intensity of the detected light at the wavelength (units: W/cm2); “αλ” represents the wavelength dependent absorption coefficient of the chromophore (units: OD*cm−1*μM−1); “C” represents the concentration of chromophore (units: μM); and “d” represents the light source to detector (optode) separation distance (units: cm).
To non-invasively determine oxygen saturation within tissue accurately, it is necessary to account for the optical properties (e.g., absorption coefficients or optical densities) of the tissue being interrogated. In some instances, the absorption coefficients or optical densities for the tissue components that create background light absorption and scattering can be assumed to be relatively constant over a selected wavelength range. The graph shown in FIG. 1, which includes tissue data plotted relative to a Y-axis of values representative of absorption coefficient values and an X-axis of NIRS light wavelength values, illustrates such an instance. The aforesaid constant value assumption is reasonable in a test population where all of the subjects have approximately the same tissue optical properties; e.g., skin pigmentation, muscle and bone density, etc. A tissue interrogation method that relies upon such an assumption may be described as being subject independent. Our findings indicate that the same assumption is not reasonable, however, in a population of subjects having a wide spectrum of tissue optical properties.
The validity of an assumption of the tissue and bone characteristics of a test population can also be questioned in instances where the tissue optical properties of tissue or bone is abnormal. One or more of disease, trauma head injury, medication, and an abnormal anatomy can cause abnormal brain and extracerebral components (scalp & skull) tissue optical properties. The abnormality of these properties can be such that conventional tissue oximetry techniques become inaccurate and difficult, and in some cases tissue oximetry algorithms cannot make a calculation at all; e.g., when the optical properties of the altered tissue (including bone) falls out of the expected range.
Examples of disease, injury, or abnormal anatomy that could result in altered tissues (e.g., cerebral and extracerebral tissue) include, but are not limited to, bruises, contusion, scarring, skull fracture, concussion, surgery scarring, sinus infections and other infections, tissue discoloration, bone discoloration, dura pigmentation, bone marrow, forehead hair follicles, etc. These conditions may be natural, or may be the result of injury or medication.
A bruise, also called a contusion, is a type of relatively minor hematoma of tissue in which capillaries and sometimes venules are damaged by trauma, allowing blood to seep into the surrounding interstitial tissues. Bruises can involve capillaries at the level of skin, subcutaneous tissue, muscle, or bone.
As a type of hematoma, a bruise is always caused by internal bleeding into the interstitial tissues, usually initiated by blunt trauma, which causes damage through physical compression and deceleration forces. Trauma sufficient to cause bruising can occur from a wide variety of situations including accidents, falls, and surgeries. Disease states such as insufficient or malfunctioning platelets, other coagulation deficiencies, or vascular disorders, such as venous blockage associated with severe allergies can lead to the formation of bruises in situations in which they would not normally occur and with only minimal trauma. If the trauma is sufficient to break the skin and allow blood to escape the interstitial tissues, the injury is not a bruise but instead a different variety of hemorrhage called bleeding, although such injuries may be accompanied by bruising elsewhere.
Increased distress to tissue causes capillaries to break under the skin, allowing blood to escape and build up. As time progresses, blood seeps into the surrounding tissues, causing the bruise to darken and spread. Nerve endings within the affected tissue detect the increased pressure, which, depending on severity and location, may be perceived as pain or pressure or be asymptomatic. The damaged endothelium (lining) of the affected capillaries releases endothelin, a hormone that causes narrowing of the blood vessel to minimize bleeding. As the endothelium is destroyed, the underlying von Willebrand factor is exposed and initiates coagulation, which creates a temporary clot to plug the wound and eventually leads to restoration of normal tissue.
During this time, larger bruises may change color due to the breakdown of hemoglobin from within escaped red blood cells in the extracellular space. The striking colors of a bruise are caused by the phagocytosis and sequential degradation of hemoglobin to biliverdin to bilirubin to hemosiderin, with hemoglobin itself producing a red-blue color, biliverdin producing a green color, bilirubin producing a yellow color, and hemosiderin producing a golden-brown color. As these products are cleared from the area, the bruise disappears. Often the underlying tissue damage has been repaired long before this process is complete. Therefore during a bruise, hemoglobin breaks down to biliverdin, then to bilirubin, to hemosiderin, which material alters the optical properties of the affected tissue. The hemoglobin breakdown to biliverdin is particularly interesting because the light absorption spectra of biliverdin shows profound light absorption as the wavelength decreases from 900 nm in the near-infrared range down to 600 nm in the red range, similar to the subject data shown in the figures of this application; e.g., See FIG. 2.
The human forehead has more hair follicles (called vellus hair) than any other part of the human body. Because most of vellus hair is fine and pale, it usually is not visible to the naked eye. The thicker, fully pigmented hair most people consider “real hair” is called terminal hair. Terminal hair is found on scalp, eyebrows, legs, backs, underarms. For most people under examination by cerebral oximetry sensor placed on the forehead, the forehead hair follicles have no effect. However for some people, the forehead hair follicle may have excessive pigmentation or other characteristics that can alter the tissue optical properties to a degree where a cerebral oximetry sensor may give an erroneous reading.
As a result of one or more of the aforementioned conditions, the optical characteristics as measured by a reflectance spectroscopy sensor, may show profound differences in the light absorption as the wavelength decreases from 900 nm in the near-infrared range down to 600 nm in the red range compared to normal subjects.